The wavelength division multiplexing (WDM) communication belongs to the optical communication technology. In the wavelength division multiplexing communication, since a multiplexed optical signal in which optical signals of a plurality of wavelengths are multiplexed is used, large-volume information can be transmitted with a single optical fiber. Further, there is a technique of selectively extracting a particular optical signal from the multiplexed optical signal, which is referred to as the coherent light transmission scheme. In the coherent light transmission scheme, by allowing the multiplexed optical signal and local oscillation light to interfere with each other and performing a coherent detection, an optical signal corresponding to the wavelength of the local oscillation light is selectively extracted from the multiplexed optical signal.
Patent Literatures 1 and 2 each disclose a technique relating to the coherent light transmission scheme. Patent Literature 1 discloses a technique of stabilizing the absolute wavelength of a local oscillation light source and a transmission light source, thereby making it easier to set the wavelength. Patent Literature 2 discloses a technique for improving the S/N ratio in the reception characteristic while suppressing an increase in costs, even in the case where a multiplexed optical signal is selectively received by the wavelength of local oscillation light.